The full-length cDNAs for human LDH-A, human LDH-B and mouse LDH- C isozymes have been expressed in E. coli. The enzymatic and non- enzymatic properties are being characterized. The cloned mouse LDH-C cDNA was inserted immediately downstream to the MMTV-5' LTR promoter, and it was shown to synthesize LDH-C polypeptide in Chinese hamster ovary cells. The mouse LDH-C subunit and the endogenous Chinese hamster LDH-A subunit formed in vitro a heterotetrameric LDH-A3C1 isozyme, and this novel isozyme exhibited enzyme activity utilizing lactate as substrate. Spermatogenic cells isolated from adult and prepubertal mice were used to examine enzyme activities and synthesis of the lactate dehydrogenase isozymes during spermatogenesis. The synthesis and activity of LDH-C4 were detected earliest in preleptotene and leptotene/zygotene spermatocytes, and were prominent in pachytene spermatocytes. Somatic-type LDH isozymes consisting primarily of LDH-B subunits were present in germ cells throughout spermatogenesis. Sertoli cells were further shown to exhibit comparable amounts of five tetrameric LDH isozymes formed by combination of LDH-A and LDH-B subunits.